WASHINGTON (CNN) -- The United States has said it does not support offering concessions to entice North Korea back to six-party negotiations on ending its nuclear program.

"The North Koreans should not be rewarded for causing difficulties in the reconvening of the talks," said U.S. State Department spokesman Richard Boucher Monday.

Three of the other parties -- China, South Korea and Japan -- have advocated a more conciliatory approach to solving the North Korean nuclear issues, and have urged the United States to be more flexible.

Boucher's comments came after a meeting Monday between U.S. Secretary of State Condoleezza Rice and South Korean Foreign Minister Ban Ki-moon.

Ban urged North Korea to return to the bargaining table "as a responsible member of the international community," and said that he and Rice "agreed to intensify our efforts among the parties concerned."

Despite North Korea's insistence it would not return to talks, Rice on Monday appointed U.S. Ambassador to Korea Christopher Hill to take on the additional role of as head of the U.S. six-party talks delegation.

Hill will inherit the envoy title from Assistant Secretary for East Asian Affairs James Kelley, who has left the State Department.

"Ambassador Hill's appointment underscores U.S. support for the six-party talks and the goal of a diplomatic solution ending North Korea's nuclear weapons programs," Boucher said.

"It was important for us to have a senior and knowledgeable person who could shepherd this work forward and move forward."

Hill will remain in Seoul, South Korea, but will be meeting with his six-party counterparts shortly, Boucher said, adding that U.S. Special Envoy Joseph DeTrani will continue to be responsible for the day-to-day oversight of issues relating to the six-party talks.

Boucher also said Monday that U.S. law enforcement agencies will aggressively work to curb illegal activities by Pyongyang, including drug trafficking, smuggling, counterfeiting and proliferation of weapons.

"We have been aware for some time of North Korea's illicit activities," Boucher said. "They have been a concern to the United States and other nations for decades," he said.

Earlier, China's foreign minister promised Washington that Beijing would push North Korea to return to the talks, a senior U.S. State Department official said.

The talks have taken on a greater sense of urgency after North Korea admitted publicly for the first time last week that it possessed nuclear weapons.

Previously it had asserted its ability and right to produce them.

In April 2003, U.S. officials said that North Koreans claimed in private meetings they had at least one nuclear bomb.

World leaders expressed concern last week after hearing that North Korea would quit the talks and "bolster its nuclear weapons arsenal."

Last Thursday's statement from the North Korean Foreign Ministry said nuclear weapons are "for self-defense to cope with the Bush administration's evermore undisguised policy to isolate and stifle" its government.

The communist state said it felt "compelled to suspend" participation in the six-nation talks "for an indefinite period."

U.S. diplomats have said that North Korea has used similar language when stepping aside from anti-nuclear proliferation talks in the past.

But it is the first time Pyongyang has been so explicit about developing nuclear weapons.